The problem that you are encountering it is really to do with humans trying to manage and control feline society which is very difficult. Legally, domestic cats can trespass on a neighbour's property as trespass doesn't apply to cats. There are some things that can be done though.
One method depends upon how often you are in your home. If you are retired and in your home most of the day you will have a chance to meet this invading cat or cats from your neighbour hundreds of yards away.
And if you can be in your backyard or garden when a large Maine Coon or large Maine Coons arrive to attack your small rescue cats, you can shoo them away.
Don't forget that humans are much larger than even Maine Coon cats and you can frighten Maine Coon cats or any other domestic cat for that matter.
A human is a very big animal compared to a Maine Coon cat and if you make a noise and wave your arms around and look threatening but don't harm the Maine Coon cat they should back off and disappear.
Ultimately, they will find coming into your garden or enclosure an uncomfortable experience and avoid it.
If you are not around to do this, you might ask a friend who is around to do it for you, but the Maine Coons must not be harmed. There should be no physical contact. It is all about sumo-style standoff displays to tell the cats that you are bigger and stronger.
An alternative would be to create a backyard or back garden enclosure which keeps your cats in, and which also keeps other cats out.
But that is an expensive method and depending upon the garden size might well cost thousands of pounds. It would provide peace of mind, however.
You can't use cat deterrents in the garden such as automatic water sprayers and any number of alternatives which are mentioned a lot on the Internet, such as ultrasound deterrents, because you will be deterring your own cats from using the same space. They are only moderately effective anyway.
I think the best method is for the human caregiver of the small rescue cats (you) to act in a hostile way towards the Maine Coons to make it clear to them that they are unwanted.
Cats learn from that, and they won't like an aggressive alien creature looking threatening at them.
But don't get too close and don't aggravate the cat to the point where they are provoked to attack you. Normally aggressive sounds and an aggressive posture by the human is enough to deter an invading or alien cat even if they are Maine Coons.
Maine Coons are normally quite retiring anyway and are not normally known to be aggressive but it will vary from individual to individual.
Good luck.